SUMMARY: Preparations of the Rothamsted culture of tobacco necrosis virus always gave two and usually three zones when centrifuged in sucrose density gradients. The top zone consisted of polyhedral particles with a sedimentation constant of 50 S, the middle zone of larger polyhedral particles with 116 S, and the bottom zone when present, consisted of the 50S particles aggregated in groups of 12. Neither the small particles (50 S) nor their aggregates were infective when inoculated either to tobacco or French bean plants, but they became so when inoculated together with the large particles (116 S). The small and large particles are serologically unrelated and seem to be different viruses, one of which depends on the other for some process that allows it to multiply to detectable amounts.

The Rothamsted culture produces local lesions of different sizes in French bean. Virus isolated from single large lesions and passed through a succession of single lesions gave, when bulked in tobacco, preparations containing up to 500 large to 1 small particle. Evidence is given which suggests that the few small particles were acquired as contaminants when the virus was bulked in tobacco, and it seems probable that small particles would not be produced in leaves infected only with the large particles. Virus obtained from small lesions when bulked gave preparations containing particles of both sizes, with ratios of large to small particles up to 1:10. Inocula of large particles produced only large lesions, whereas mixed inocula produced large and small lesions, in proportions which depended on the ratio of the two kinds of particles in the inoculum. Two different tobacco necrosis viruses activated the small particles, but tobacco mosaic and some other viruses did not.

Particles of the two activating viruses differed in size and in their stability when negatively stained in phosphotungstate. When fixed and negatively stained they appeared to be angular, but much less so than when shadowed. Small particles tended to pack in regular arrays, and one type of packing suggested that they have a fivefold axis of symmetry.


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